Re-configurable SLMs based on liquid crystal (and other types of) devices are widely used for controlling and manipulating optical beams. In diffractive mode they may be used for three dimensional (3D) imaging [BROWN, C V and STANLEY, M, UK Patent Application GB2330471, Production of Moving Images for Holography] and for routing optical signals in telecommunications networks [See for example ROSES (Re-configurable Optical Switches) website, http://www-g.eng.cam.ac.uk/photonics/rose1.html].
The SLM.modulates the complex amplitude of an incoming wave front (i.e. changes its phase and/or amplitude), which causes it to propagate in the desired manner. The SLM generally comprises a liquid crystal panel containing a number of individually addressed pixels, onto which a diffraction pattern or Computer Generated Hologram (CGH) is written [CAMERON, C D et al, SPIE Conference on Critical Technologies for the Future of Computing (San Diego, USA), July-August 2000, Computational Challenges of Emerging Novel True 3D Holographic Displays]. CGH 3D display systems typically use a computer to generate and/or store electronic copies of the hologram. This hologram is then replayed on an SLM which is switched to modulate (in transmission or reflection) light from a source which then passes through suitable replay optics, thereby providing a visible three-dimensional image to observers.
For many image generation applications, especially holographic 3D image generation, it is important to maximise the image size and/or the range of angles over which the image can be viewed. Conventionally, this is achieved by increasing the spatial frequency content of the hologram, which increases the achievable diffraction angle of the modulated beam and/or increasing the number of pixels in the SLM. In order to produce satisfactory images, SLMs containing of the order of 10.sup.10 pixels may be required. An increased spatial frequency is also desirable for many other applications utilising diffractive SLMs, such as optical switching.
HAINES and BRUM [Proceedings of the IEEE (Letters), p 1512-3, August 1967, A Technique for Bandwidth Reduction in Holographic Systems] proposed using a ground glass scatter plate as a means of enhancing the viewing angle without having to increase the spatial frequency content. The principle was successfully demonstrated using a conventional fixed, photographically recorded hologram of a point source hologram (note: the scatter plate is used in both the recording and reconstruction stage). KOMAR [SPIE Vol 120, p 127, 1977, Progress on the Holographic Movie Process in the USSR] discussed the use of the scatter-plate technique as a means of producing holographic “movies” in which the viewing angle was big enough for a large audience to see the 3D images. He concluded that the technique caused unacceptable degradation in the image quality.
The main factor preventing the introduction of reconfigurable computer generated holograms (CGH) in many applications is the number of addressable, reconfigurable pixels required in such devices. This is particularly important in the area of 3D image generation.
To ensure adequate fields of view (FOV) and image sizes, conventional approaches typically need a CGH having a pixel count several orders of magnitude higher than that required to produce image resolutions that the human visual system can perceive. The simple relation FOV.1.about.n.lamda./4 (1) (where n is the number of pixels across the display in the plane where the FOV is specified, I is image width, and A is the wavelength of light generating the real image) shows that, for typical applications (e.g. FOV=.+−.30.degree., I=0.05 m), .about.10.sup.10 pixels are required to be addressed. This number is enormous. Any method allowing a pixel count reduction, without significantly compromising perceived image qualities, will have great effect on the practicality of such systems.